This invention relates to a data transmission system and more particularly to a system having improved reliability.
FIG. 1 shows a conventional bus type optical data transmission system. In FIG. 1, each of nodes 12-17 has two sets of optical transmitters and receivers, transmitting and receiving in opposite directions. A node which is not transmitting converts all the optical signals it has received into electrical signals. After that, it converts them back into optical signals and transmits these signals to the neighbouring nodes.
Two optical transmission lines, arranged in two sets for opposite direction of transmission, function like a single electrical transmission cable.
The following problems are associated with a system such as that shown in FIG. 1.
(1) When an intermediate node loses its transmission function because of some system fault, the nodes on its right and left become separated. PA1 (2) When there is a break at one place in an optical transmission line, or one optical transmitter or optical receiver is out of action, transmission is then possible only in one direction. PA1 (1) When one intermediate node ceases to be able to transmit left and right, transmission between the remaining nodes becomes impossible. PA1 (2) If transmission becomes impossible in either the left or the right direction, as when a fault occurs in a transmission line, transmitter or receiver of an intermediate node, and this fault is incompletely isolated, the nodes are divided into two separate groups, to the left and right of the point where the fault has occurred, and transmission is impossible between these groups.
For example, if there is a fault in node 15 transmission between nodes 12 -14 and nodes 16 -17 becomes impossible.
In this event, any fault between nodes should preferably be isolated immediately and transmission should be continued within the group of nodes thus isolated.
In a conventional system, isolation when a fault occurs is effected by a single special node known as the supervisor node, which oversees the whole system. When a fault occurs, this supervisor node temporarily suspends normal transmission, sends a frame to the next node as a check, checks its response, and then repeats this routine with the next node along the line.
This enables it to decide which node, or which line between which nodes, is not functioning properly. Having made this decision, the supervisor node isolates the fault by issuing a command to the relevant nodes, forbidding them to receive or transmit.
For example, if the supervisor node is the leftmost node 12, and a break occurs in optical transmission line 20, supervisor node 12 first checks the link with node 13, and then checks nodes 14 and 15. Nodes 15 and above do not respond, because they cannot receive signals from node 12 owing to the break in optical transmission line 20. The supervisor node therefore issues a command to node 14 forbidding it to receive from the right. The purpose of this is to prevent transmission between nodes 12-14 being disturbed by transmission from nodes 15 and up. The time of this checking is proportionate to the number of nodes involved, and isolation of the fault becomes a long process. Transmission cannot take place between nodes 15-17, which cannot be overseen by the supervisor node, since usually there is only one such supervisory node. A further supervisory node could be inserted between nodes 15 and 17, but if there are two supervisory nodes operating at the same time, data from nodes 15 -17 will be sent via optical transmission line 26 to nodes 12 -14, and the checking from the two sides will collide, making the operation more difficult. The key point is the difficulty, when supervision is by one node only, of continuing transmission within a plurality of groups when a fault has developed.
The following is a summary of the problems associated with the conventional optical data transmission system.